How to unlink a linked-duplicate?
You can press U &gt; Unlink Object+Data. The object's &quot;data&quot; is the mesh itself. Multiple objects can use a single mesh, so that editing one will update the others (they're &quot;Linked&quot;)
You could also go into the Object Data tab and manually select different mesh data for the object to use:

The 2 on the right indicates that that mesh is being used by two objects. Clicking on the 2will copy that mesh and unlink that object from the old mesh, linking it to the new one (using an incremented name like Cube.001), the same result as if you'd done U &gt; Unlink Object+Data
I know you already know the answer, but this is a place for the future too :)